The Global Positioning System ("GPS") was conceived, developed, and deployed by the United States Department of Defense over ten years ago. The GPS' purpose was to provide air, ground, and naval users with precise position and time information. Although principally conceived for military purposes, the GPS also includes a band of signals designated for non-military use. These non-military signals may be used advantageously in numerous private and commercial applications. For example, the GPS can be used by commercial airlines and private pilots, to accurately and quickly ascertain their location. Similarly, both commercial shippers and private boatmen can use the GPS at any time to accurately determine their position, without the need for conventional points of reference, such as the magnetic poles.
The GPS is a series of twenty-four satellites orbiting the earth. The satellites are deployed in twelve hour orbits and are dispersed in six orbital planes. These satellites continuously emit electronic signals--telemetry--which are received by ground-based or other terrestrial receivers. By simply receiving the signal from four or more orbiting GPS satellites, a properly configured receiver can accurately determine the position of a vehicle carrying the receiver, the vehicle's velocity, the GPS satellite's position, and the time. In addition, technology has been developed to determine information relating to the attitude, direction of travel, pitch, and roll of a vehicle carrying the receiver from the GPS signals. This technology is fully disclosed in applicant's co-pending U.S. application Ser. No. 08/077,734 entitled "Attitude Determining System For Use With Global Positioning System", the disclosure of which is incorporated herein by reference. The Attitude Determining System ("ADS") provides heading and navigation information to naval, air borne, or ground-based vehicles.
The GPS ADS uses carrier-phase interferometry of the GPS signals to measure the signal arrival time difference at two or three closely separated antennas. Carrier phase interferometry refers to measuring the difference in the time that is takes a satellite signal to be received by two or more antenna spaced a known distance apart. By using the GPS satellite signal arrival time differences, it is possible to determine which antenna is closest to each satellite. This arrival time difference can be used to calculate an angle between each GPS satellite and a baseline formed by each antennae pair. These angles, combined with the position of each satellite and of the ADS system to calculate the baseline attitude. The calculations are performed using an algorithm developed by applicant and disclosed in the aforementioned U.S. application Ser. No. 08/077,734.
The GPS has tremendous benefits over other positioning systems, such as gyroscopes, magnetic compasses, and theodolites, in that it relies upon no visual, magnetic, or other point of reference. These advantages are particularly important in applications such as aviation and naval navigation which traverse polar regions where conventional magnetic navigational means are rendered less effective by local magnetic conditions. Moreover, there is no signal degradation, or loss of quality of positioning information at higher latitudes. Magnetic deviations and anomalies common in standard positioning systems do not exist. In addition, the GPS is typically fabricated of standard, solid state electronic hardware, resulting in a low cost, low maintenance system, having few or no moving parts, and requiring no optics. The GPS does not have the calibration, alignment, and maintenance requirements of conventional inertial measuring units. Also, the GPS is available twenty-four hours a day on a worldwide basis. Further, normal GPS operation provides positioning with accuracies of about 100 meters for non-military uses. Military users can make use of encryption keys which yield position information within 10 meters.
Notwithstanding these advantages, a need arose for a mobile system capable of determining the location of selected communication satellites at any given time, then determining the pointing angle to a selected earth-orbiting satellite, such as a communications satellite. Prior devices for determining satellite location and position were generally integrated into larger communications systems. So, to determine if satellite communication was possible at a given location, the entire communications system had to be assembled before satellite location was ascertainable. As a consequence, oftentimes personnel invested energy and time in assembling the satellite communications system only to find the communications path to a selected satellite obstructed by, for example, a tree or hilly terrain. Thus, a device capable of quickly ascertaining the precise location and pointing angle to selected orbiting satellites, while eliminating the need to assemble the entire communications system, became desirable.
The subject satellite locating and pointing capability has particular utility for military forces engaged in covert operations in disputed territory, although nonmilitary applications may also be envisioned. Moreover, the development of new technological applications of the GPS was limited by the structural size of prior art communication and navigation devices. Thus, a more compact, portable device capable of interacting with GPS satellites and other earth-orbiting satellites, particularly communications satellites, became desirable.
These deficiencies and other limitations of conventional navigation and satellite location systems are addressed by the invention disclosed herein. These and other objects and advantages of the subject invention will become apparent from a perusal of the Detailed Description of the Invention, the Drawings, and the Claims which follow.